Superharmonic imaging uses higher harmonics like third and fourth harmonic to increase the contrast to tissue ratio compared to a second harmonic imaging mode. Second harmonic imaging is better than fundamental imaging, but has limited capabilities to discriminate between tissue and microbubbles, caused by the non-linear propagation of ultrasound.

An imaging direction, upwards relative to the long axis of the human body.

The international system for units.
Le Systeme international d'Unites officially came into being in October 1960 and has been adopted by nearly all countries, though the amount of actual usage varies considerably.
It is based upon 7 principal units:
Length - meter (m)
Mass - kilogram(kg)
Time - second (s)
Electric current - ampere (A)
Temperature - Kelvin (K)
Amount of substance - mole (mol)
Luminous intensity - candela (cd)
From these basic units many other units are derived and named.

See also Ultrasound Physics.